Monthly wind directional indices based on ships logbooks observations, NI

SOURCES:
Daily data of wind direction over the English Channel based on direct marine observations derived from Royal Navy ships' logbooks (1685-1870) and ICOADS v3.0 (1750-2014) datasets.
DESCRIPTION:
The NI is the Directional Indices (DIs) that give a measure of the notherly flow. It is an instrumental-based monthly index defined as the proportion of notherly days per month (in % with respect to the total number of available days for each month)over the English Channel.
A notherly day is defined as a day with prevailing wind from the east (wind blowing from the [315, 45)N sector). When only one daily observation was available, the day was classified from the wind direction of that single observation. If several observations per day were available overthe English Channel, a notherly wind day was said to occur when at least a percentage of 36% of the total number of observations of that day corresponded to westerly wind.
DATA:
NI: Notherly Index (% of days in a month with notherly wind direction with respect to the total number of available (i.e., non missing) days in the month)
No: Number of available days (i.e., with at least one wind observation) in a month (in % with respect to the total number of days in the month). Months with no wind direction observation at all in the full month are labeled as missing (-99.9)
IMPORTANT NOTE: Please, note that the NI is herein provided regardless the number of observations. Some months have a low number of observations (No < 50 %)